1. Administration of an aqueous extract of the dried leaves of Solanum malacoxylon (DLSM) to rats causes a rapid hyperphosphataemia and a decrease in plasma alkaline phosphatase activity; the two effects are typical of 1,25(OH)2D3, the hormonally active metabolite of vitamin D3. 2. DLSM, like both vitamin D3 and parathyroid hormone, increases plasma calcium and citrate levels in rats. The effect of DLSM in influencing plasma citrate, and the role of this important metabolite in mineral metabolism is discussed. 3. A decrease of plasma magnesium levels occurs in rats following treatment with DLSM. This decrease, which is associated with a renal loss of this cation, is remarkably similar to that produced by hypervitaminosis D3. 4. Prolonged administration of DLSM to vitamin D deficient rats causes a polyuria, hypercalciuria, hyperphosphaturia, hypermagnesuria, an increase in urinary total hydroxyproline, an increase in plasma total hexosamines, and a corresponding decrease in the bone total hexosamines. These effects, some of which can also be produced by hyperparathyroidism, or following the administration of parathyroid extract (PTE), large doses of vitamin D3, or 1,25(OH)2D3, suggest that DLSM, like the latter compounds, is capable of causing bone mineral mobilization, and the dissolution of bone organic matrix.